The Luhn algorithm also known as the “modulus 10″ or “mod 10″ algorithm, is a checksum formula which can be used to validate credit card numbers. Developed in the 1950′s by IBM scientist Hans Peter Luhn and described in U.S. Patent 2,950,048. A PHP implementation is shown below.   function LuhnCheck($strDigits) { $sum = 0; [...]